Method for producing propylene oxide

ABSTRACT

Process for preparing propylene oxide, which comprises at least the steps (iii) and (iv) (iii) separating off propylene oxide from a mixture (M1) comprising propylene oxide and at least one solvent by distillation in a distillation column, giving a bottom stream and a vapor stream consisting essentially of propylene oxide; (iv) compressing the vapor stream obtained in (iii) by means of at least one compressor to give a compressed vapor.

The present invention relates to a process for preparing propyleneoxide, in which a mixture consisting essentially of propylene oxide andat least one solvent, preferably methanol, is worked up by distillation.In this work-up, a vapor from the distillation column, which consistsessentially of propylene oxide, is compressed to give a gaseous vaporwhich is under a particular pressure. In a preferred embodiment, theenergy contained in the compressed vapor is at least partly returned tothe distillation process, for example for operating at least onevaporizer of the distillation column. In further preferred embodiments,the present invention encompasses further aspects which have a positiveeffect on the energy balance of the overall process.

In the numerous publications on the subject of the preparation ofpropylene oxide, there are only a few which are concerned withintegrated processes in which the energy of the vapor obtained in adistillation step is usefully returned to the process. This appliesparticularly to processes in which propylene oxide is separated off fromsolvents or traces of solvent by distillation.

WO 02/14298 A1 describes a process for the continuous preparation of anolefin oxide. In the context of this process step, it is disclosed thatthe heat of condensation obtained at the top of a column can berecovered for one or all distillation processes of the overall process.In the column in question, a mixture comprising solvent, oxygen andinert gas is separated by distillation. Specific procedures forrecirculating the heat of condensation are not disclosed.

WO 00/07965 describes a process for preparing propylene oxide, in whicha mixture of propene, propylene oxide and methanol is separated off froma mixture via the top of a distillation column, with the refluxnecessary for the separation in the column being condensed in a partialcondenser at the top of the column.

If methanol, for example, is used as solvent in the preparation ofpropylene oxide from propene, it is generally advantageous for this tobe used in the reaction section, i.e. for the reaction of propene with ahydroperoxide such as hydrogen peroxide, particularly when a titaniumsilicalite catalyst of the TS-1 type is used as catalyst for thereaction. On the other hand, the presence of methanol makes purificationof the propylene oxide more difficult.

At atmospheric pressure or superatmospheric pressures, essentially inthe range from 1 to 5 bar, propylene oxide and methanol can be separatedby distillation only when a distillation column having a very largenumber of theoretical plates is used and a very high reflux ratio is setat the same time, owing to the entraining azeotrope.

The separation task is simpler at lower pressures, but the low pressurehas an adverse effect on the condensation temperature since thecondensation temperature, which can, for example, be in the region of15° C. depending on the pressure, requires provision of a highrefrigeration power for condensation. Especially on an industrial scale,this incurs tremendous costs.

It is an object of the present invention to provide a process which,compared to the processes described in the prior art for preparingpropylene oxide, has a significantly improved energy balance.

The present invention accordingly provides a process for preparingpropylene oxide, which comprises at least the steps (iii) and (iv)

-   (iii) separating off propylene oxide from a mixture (M1) comprising    propylene oxide and at least one solvent by distillation in a    distillation column, giving a bottom stream and a vapor stream    consisting essentially of propylene oxide;-   (iv) compressing the vapor stream obtained in (iii) by means of at    least one compressor to give a compressed vapor.

The solvent or solvents present in the mixture (M1) in (iii) can inprinciple be any solvent(s) which is/are used in the process forpreparing propylene oxide.

Particularly preferred solvents are, for example:

-   -   water;    -   alcohols, preferably alcohols having less than 6 carbon atoms,        more preferably methanol, ethanol, propanols, butanols,        pentanols;    -   diols or polyols, preferably those having less than 6 carbon        atoms;    -   ethers such as diethyl ether, tetrahydrofuran, dioxane,        1,2-diethoxyethane, 2-methoxyethanol;    -   esters such as methyl acetate or butyrolactone;    -   amides such as dimethylformamide, dimethylacetamide,        N-methylpyrrolidone;    -   ketones such as acetone;    -   nitrites such as acetonitrile;    -   or mixtures of two or more of the compounds mentioned.

Very particular preference is given to using a mixture (M1) comprisingpropylene oxide together with methanol or water or methanol and water assolvent in the process of the present invention. The mixture (M1) morepreferably comprises methanol as solvent.

Accordingly, the present invention provides a process as described abovein which the at least one solvent is methanol.

The solvent content and particularly preferably the methanol content ofthe propylene oxide separated off from the mixture (M1) in (iii) isgenerally not more than 500 ppm, preferably not more than 200 ppm, morepreferably not more than 100 ppm, more preferably not more than 50 ppm,more preferably not more than 20 ppm, particularly preferably not morethan 15 ppm and very particularly preferably not more than 10 ppm, ineach case based on the total weight of the propylene oxide fractionseparated off.

As distillation column, it is essentially possible to use any column.Particular preference is given to a distillation column configured as apacked column, more preferably a packed column containing orderedpacking. Such a packed column has a high separation efficiency per meterof packing and displays only a very small pressure drop.

While the ordered packing mentioned can essentially be of any type,preference is given to packing which has a specific surface area in therange of from 100 to 750 m²/m³. It is possible to use sheet metalpacking, for example from Montz (type B1 100 to B1 500) or from SulzerChemTech (Mellapak 125 to Mellapak 750), or mesh packing from Montz(type A3 500 to A3 750) or from Sulzer ChemTech (type BX or CY). Theunit m²/m³ refers to the geometric surface area of the material formingthe packing per cubic meter of packing.

While the distillation in (iii) can generally be carried out under allsuitable conditions, according to the present invention preference isgiven to embodiments of the distillation in (iii) in which the mixture(M1) is fractionated under reduced pressure. For the purposes of thepresent invention, the term “distillation under reduced pressure” refersto any distillation which is carried out at a pressure of less than1.013 bar.

The distillation in (iii) is therefore generally carried out atpressures of less than 1.013 bar, preferably in a range up to 1 bar,more preferably in a range of from 300 to 900 mbar, more preferably in arange of from 400 to 800 mbar and particularly preferably in a range offrom 450 to 750 mbar.

Accordingly, the present invention also provides a process as describedabove in which the distillation column used for the fractionaldistillation in (iii) is operated at a pressure in the range of from 450to 750 mbar.

In step (iv) of the process of the present invention, the vapor obtainedat the top of the distillation column, which consists essentially ofpropylene oxide, is compressed. This compression can generally becarried out using any suitable methods. In particular, the vapor can becompressed mechanically or thermally, with compression being able to becarried out in one or more apparatuses. It is thus possible to compressthe vapor mechanically in at least one compression apparatus or tocompress the vapor thermally in at least one compression apparatus orfirstly to compress the vapor mechanically in at least one compressionapparatus and then to compress the vapor thermally in at least onecompression apparatus or firstly to compress the vapor thermally in atleast one compression apparatus and then to compress the vapormechanically in at least one compression apparatus.

Apparatuses suitable for mechanical compression are, for instance,rotary piston compressors, screw compressors, turbocompressors having anaxial or radial construction, diaphragm-type compressors or blowers. Forthe purposes of the present invention, compression can be carried outusing one of these apparatuses or a combination of two or more of theseapparatuses, with each of the compressors used being able to have one ormore stages.

An example of an apparatus for thermal compression is a steam ejectorwhich can be equipped with a fixed or regulatable driving nozzle.

For the purposes of the present invention, the vapor is particularlypreferably compressed mechanically, once again preferably in a singleapparatus. Preference is in turn given to a turbocompressor, veryparticularly preferably a single-stage single-screw compressor.

Accordingly, the present invention also provides a process as describedabove in which compression of the vapor is carried out using aturbocompressor.

In a more preferred embodiment of the process of the present invention,the vapor is compressed by means of the preferred mechanical compressorso that the vapor has a pressure in the range of generally from 1.5 to 5bar, preferably from 2 to 4 bar and particularly preferably from 2.5 to3.5 bar, after leaving the compressor.

In general, the vapor is brought by compression to a temperature whichis at least 1° C. higher than the temperature of the medium vaporizingin the bottom of the distillation column. The vapor is preferablybrought by compression to a temperature which is from 2 to 25° C., morepreferably from 5 to 20° C. and particularly preferably from 8 to 20°C., higher than the temperature of the medium vaporizing in thedistillation column.

Accordingly, the present invention also provides a process as describedabove in which the vapor is compressed to a pressure in the range offrom 2 to 5 bar in (iv) and the compressed vapor has a temperature whichis in a range of from 8 to 20° C. above the temperature of the mediumvaporizing in the distillation column in (iii).

As a result of the compression step according to the present invention,the process of the present invention makes possible the above-describedfavorable pressure range below 1.013 bar, preferably the range from 450to 750 mbar, for the distillation without having to accept thedisadvantage of a low condensation temperature and the highrefrigeration power which then has to be made available.

Depending on the composition of the feed and the required purity of thepropylene oxide in respect of the residual concentration of solvent,preferably methanol, the compressor power is in the range of from 3 to 9MW. The corresponding condensation/refrigeration power which would haveto be employed at a temperature in the range of from 12 to 20° C. wouldbe in a range of from 15 to 25 MW.

The energy additionally stored in the vapor as a result of compressioncan, for example, preferably be fed to any process, with recirculationinto the process of the present invention being preferred. In general,all or part of the quantity of energy can be introduced into any processstep. Particular preference is given to recirculation of at least partof the energy stored in the compressed vapor to the distillation step(iii). Particular preference is in this case given to at least onevaporizer of the distillation column, for example at least oneintermediate vaporizer or the main vaporizer or at least oneintermediate vaporizer and the main vaporizer, being operated by meansof the energy stored in the compressed vapor. In this way, a heat pumpis realized in the process of the present invention as a result of thisintegrated operation of the process.

In a very particularly preferred embodiment, the compressed gaseousvapor is liquefied in at least one condenser and the heat ofcondensation is at least partly used for operating at least one of theabovementioned vaporizers. Particular preference is given to theoperation of the main vaporizer of the distillation column used in(iii).

Accordingly, the present invention also provides a process as describedabove which additionally comprises the step (v)

-   (v) condensing the vapor obtained in (iv) and returning at least    part of the heat of condensation to at least one vaporizer used in    the distillation column employed in (iii).

The condensation in (v) is carried out in a vaporizer which can haveessentially any configuration. Examples of embodiments of vaporizers arenatural convection vaporizers, forced circulation vaporizers or fallingfilm vaporizers. For the purposes of the present invention, preferenceis given to using a vaporizer which is configured as a naturalconvection vaporizer.

The cooled condensate leaving the condenser or condensers of (v) has, inthe process of the present invention, a temperature of generally from 40to 75° C., preferably from 45 to 70° C. and particularly preferably from45 to 65° C.

It is generally possible to operate the vaporizer using only the energyrecovered as described above from the condensation of the compressedvapor. In a more preferred embodiment of the process of the presentinvention, at least one further vaporizer which serves to compensatedifferences in the condensation and vaporization enthalpy andaccordingly functions as a supplementary or secondary vaporizer isadditionally provided.

This/these additional vaporizer or vaporizers can have essentially anyconfiguration. Examples of embodiments of the additional vaporizer orvaporizers are natural convection vaporizers, forced circulationvaporizers or falling film vaporizers. For the purposes of the presentinvention, preference is given to using a vaporizer which is configuredas a natural convection vaporizer.

In a more preferred embodiment of the process of the present invention,at least part of the condensate obtained in (v) is cooled further in atleast one further heat exchanger so as to obtain energy which can bepassed to any other process or preferably be recirculated within theprocess of the present invention.

This part of the condensate is preferably cooled in the further heatexchanger or exchangers to a temperature in the range from 10 to 30° C.,particularly preferably in the range from 12 to 20° C.

In a very particularly preferred embodiment, the cooled condensateleaving this heat exchanger or exchangers is returned as reflux to thedistillation column used in (iii).

Accordingly, the present invention also provides a process as describedabove which additionally comprises the step (vi)

-   (vi) cooling at least part of the condensate obtained in (v) to a    temperature in the range from 10 to 30° C. in at least one heat    exchanger and returning this part of the cooled condensate as reflux    to the distillation column used in (iii).

In the process of the present invention, the refrigeration poweremployed in the heat exchanger of (vi) for cooling the condensate ispreferably provided by at least a part of the process of the presentinvention. For example, it is conceivable for the refrigeration powerrequired in the heat exchanger of (vi) to be taken from a refrigerantwhich at another point of the process once again takes up the quantityof cold withdrawn in this way. However, it is also conceivable for therefrigeration power taken up in the heat exchanger to be transferreddirectly from a material or mixture which can generally be in anypossible state of matter. For example, preference is given, in theprocess of the present invention, to depressurizing a compressed streaminto a compartment of the heat exchanger and at least partly, preferablycompletely, vaporizing it and transferring the resulting refrigerationpower to the condensate present in another compartment of the heatexchanger. Preference is in turn given to an embodiment in which thiscompressed stream is a compressed propene stream. In particular, thispropene stream is a compressed propene stream which is firstly, asdescribed above, depressurized into the heat exchanger and vaporized inthe heat exchanger and is subsequently used in step (i), which isdescribed below, of the process of the present invention.

The compressed propene stream is particularly preferably vaporizedcompletely in the vaporizer or vaporizers used in (vi).

Accordingly, the present invention also provides a process as describedabove in which the propene compressed in the vaporizer or vaporizersused in (vi) is vaporized completely with depressurization.

For example, the propene stream has preferably been compressed to apressure in the range from 20 to 35 bar at a temperature in the rangefrom 5 to 30° C., preferably from 10 to 30° C., more preferably from 15to 30° C. and particularly preferably from 20 to 30° C., and is,according to the present invention depressurized in step (vi) to apressure in the range from 4 to 10 bar, preferably from 5 to 9 bar andmore preferably from 5 to 8 bar, and vaporized completely byintroduction of heat. For example, about half the cold of expansion ofthe propene is produced by means of this step.

The depressurization of the compressed stream in (vi) is into a heatexchanger which can have essentially any configuration. Examples ofconfigurations of the heat exchanger are shell-and-tube heat exchangers,coil heat exchangers or plate heat exchangers. For the purposes of thepresent invention, preference is given to using a heat exchanger whichis configured as a shell-and-tube heat exchanger.

The bottom stream obtained from (iii) can, according to a more preferredembodiment of the process of the present invention, likewise be used forimproving the energy integration of the process of the present inventioneven further.

For this purpose, the quantity of heat contained in the bottom streamobtained from (iii) is at least partly used for heating the mixture (M1)before it is introduced into the distillation column in (iii).Particular preference is here given to using a heat exchanger configuredas a countercurrent heat exchanger (plate heat exchanger).

Accordingly, the present invention also provides a process as describedabove in which the energy stored in the bottom stream obtained from(iii) is at least partly used for heating the mixture (M1) before it isfractionally distilled in (iii).

The mixture (M1) used in (iii) results from essentially any processsteps in the preparation of propylene oxide, provided that a mixture(M1) as described above is obtained.

The mixture (M1) is particularly preferably obtained from a process inwhich propene is reacted with a hydroperoxide in methanol as solvent inthe presence of a zeolite catalyst.

In the process of the present invention, the propene is reacted with atleast one hydroperoxide. For the purposes of the present patentapplication, the term “hydroperoxide” refers to a compound of theformula ROOH. Details regarding the preparation of hydroperoxides andregarding hydroperoxides which can be used, inter alia, in the processof the present invention may be found in DE-A 198 35 907, whose relevantcontents are incorporated by reference into the disclosure of thepresent patent application. Examples of hydroperoxides which can be usedaccording to the present invention include tert-butyl hydroperoxide,ethylbenzene hydroperoxide, tert-amyl hydroperoxide, cumenehydroperoxide, cyclohexyl hydroperoxide, methylcyclohexyl hydroperoxide,tetrahydronaphthalene hydroperoxide, isobutylbenzene hydroperoxide,ethylnaphthalene hydroperoxide, peracids such as peracetic acid orhydrogen peroxide. Mixtures of two or more hydroperoxides can also beused according to the present invention. For the purposes of the presentinvention, preference is given to using hydrogen peroxide, morepreferably an aqueous hydrogen peroxide solution, as hydroperoxide.

The zeolite catalysts which can be used for the purposes of the presentinvention are subject to no particular restrictions.

It is known that zeolites are crystalline aluminosilicates havingordered channel and cage structures and containing micropores which arepreferably smaller than about 0.9 nm. The network of such zeolites ismade up of SiO₄ and AlO₄ tetrahedra which are joined via shared oxygenbridges. An overview of known structures may be found, for example, inW. M. Meier, D. H. Olson and Ch. Baerlocher, “Atlas of Zeolite StructureTypes”, Elsevier, 5th edition, Amsterdam 2001.

Zeolites which contain no aluminum and in which part of the Si(IV) inthe silicate lattice is replaced by titanium as Ti(IV) are also known.These titanium zeolites, in particular those having a crystal structureof the MFI type, and possible ways of preparing them are described, forexample, in EP-A 0 311 983 or EP-A 0 405 978. Apart from silicon andtitanium, such materials can further comprise additional elements suchas aluminum, zirconium, tin, iron, cobalt, nickel, gallium, germanium,boron or small amounts of fluorine. In the zeolite catalysts thetitanium of the zeolite can be partly or completely replaced byvanadium, zirconium, chromium or niobium or a mixture of two or morethereof. The molar ratio of titanium and/or vanadium, zirconium,chromium or niobium to the sum of silicon and titanium and/or vanadiumand/or zirconium and/or chromium and/or niobium is generally in therange from 0.01:1 to 0.1:1.

Titanium zeolites, in particular those having a crystal structure of theMFI type, and possible ways of preparing them are described, forexample, in WO 98/55228, EP-A 0 311 983 or EP-A 0 405 978, whoserelevant contents are fully incorporated into the disclosure of thepresent patent application.

It is known that titanium zeolites having an MFI structure can beidentified via a particular X-ray diffraction pattern and additionallyvia a lattice vibration band in the infrared region (IR) at about 960cm⁻¹ and in this way differ from alkali metal titanates or crystallineand amorphous TiO₂ phases.

Specific mention may be made of titanium-, germanium-, tellurium-,vanadium-, chromium-, niobium-, and zirconium-containing zeolites havinga pentasil zeolite structure, in particular the types which can beassigned X-ray-crystallographically to the ABW, ACO, AEI, AEL, AEN, AET,AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST,ATN, ATO, ATS, ATT, ATV, AWO, AWW, BEA, BIK, BOG, BPH, BRE, CAN, CAS,CFI, CGF, CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO, DFT, DOH, DON,EAB, EDI, EMT, EPI, ERI, ESV, EUO, FAU, FER, GIS, GME, GOO, HEU, IFR,ISV, ITE, JBW, KFI, LAU, LEV, LIO, LOS, LOV, LTA, LTL, LTN, MAZ, MEI,MEL, MEP, MER, MFI, MFS, MON, MOR, MSO, MTF, MTN, MTT, MTW, MWW, NAT,NES, NON, OFF, OSI, PAR, PAU, PHI, RHO, RON, RSN, RTE, RTH, RUT, SAO,SAT, SBE, SBS, SBT, SFF, SGT, SOD, STF, STI, STT, TER, THO, TON, TSC,VET, VFI, VNI, VSV, WIE, WEN, YUG, ZON structure or to mixed structurescomprising two or more of the abovementioned structures. Furthermore,titanium-containing zeolites having the ITQ-4, SSZ-24, TTM-1, UTD-1,CIT-1 or CIT-5 structure are also conceivable for use in the process ofthe present invention. Further titanium-containing zeolites which may bementioned are those of the ZSM-4 or ZSM-12 structure.

For the purposes of the present invention, particular preference isgiven to using Ti zeolites having an MFI or MEL structure or an MFI/MELmixed structure. Further preference is given to the specificTi-containing zeolite catalysts which are generally referred to as“TS-1”, “TS-2”, “TS-3” and also Ti zeolites having a framework structureisomorphous with beta-zeolite. For the purposes of the presentinvention, very particular preference is given to a zeolite catalyst ofthe TS-1 type.

After the reaction of the propene with, preferably, hydrogen peroxide togive a mixture (M0) comprising propylene oxide, unreacted propene andmethanol, unreacted propene is preferably separated off from thismixture (M0), preferably by distillation.

Accordingly, the present invention also provides a process as describedabove which additionally comprises the steps (i) and (ii)

-   (i) reacting propene with hydrogen peroxide in the presence of a    titanium silicalite catalyst and methanol as solvent to give a    mixture (M0) comprising propylene oxide, unreacted propene and    methanol;-   (ii) separating off the unreacted propene from the mixture (M0) to    give a mixture (M1) comprising propylene oxide and methanol.

The reaction of propene with hydrogen peroxide in the presence ofmethanol and the titanium silicalite catalyst can be carried out in one,two or more stages, particularly preferably in two stages.

A two-stage reaction takes place, for example, as follows:

-   (a) the hydrogen peroxide is reacted with propene to give a mixture    comprising propylene oxide and unreacted hydrogen peroxide;-   (b) the unreacted hydrogen peroxide is separated off from the    mixture resulting from stage (a);-   (c) the hydrogen peroxide which has been separated off in stage (b)    is reacted with propene.

Accordingly, the reaction of propene with hydrogen peroxide takes place,as indicated, in two stages (a) and (c), with a separation stage (b) inbetween.

For the purposes of the present invention, the hydrogen peroxide can beseparated off in the separation stage (b) using all customary separationmethods known from the prior art.

The hydrogen peroxide is preferably separated off by distillation.Depending on the requirements of the process, it can be separated off inone or more distillation columns. Preference is given to using onedistillation column in a separation stage.

In the process of the present invention, the reaction of propene withhydrogen peroxide takes place in a reactor which is suitable for thispurpose. Starting materials used for the reaction are propene, hydrogenperoxide and methanol. In this process, the starting materials can befed individually into the reactor or are preferably combined to form onestream and fed in this form into the reactor. In the process of thepresent invention, preference is given to feeding a stream consisting ofthe combination of the starting materials into the reactor. Preferenceis in this case given to a stream in which the concentrations of theindividual starting materials in the stream are selected so that thestream is liquid and consists of a single phase.

In a further, preferred embodiment, it is possible to carry out thereaction in stages (a) and (c) in two separate reactors.

As reactors, it is of course possible to use all conceivable reactorswhich are best suited to the respective reaction. In this context, theterm “a reactor” is not restricted to a single vessel. Rather, it isalso possible to use a cascade of stirred vessels as reactor.

Preference is given to using fixed-bed reactors as reactors. Greaterpreference is given to using fixed-bed tube reactors as fixed-bedreactors, with at least one of the reactors also being able to beoperated in the suspension mode.

In the case of the reactions in stages (a) and (c) being carried out intwo separate reactors, particular preference is given to using oneisothermal fixed-bed tube reactor and one adiabatic fixed-bed reactor.Preference is given to using the isothermal fixed-bed tube reactor instage (a) and the adiabatic fixed-bed reactor in stage (b).

In a preferred embodiment of the process of the present invention, themixture (M1) comprises from 5 to 15% by weight, preferably from 6 to 12%by weight and particularly preferably from 8 to 10.5% by weight, ofpropylene oxide and from 55 to 85% by weight, preferably from 60 to 80%by weight and particularly preferably from 65 to 75% by weight, ofmethanol.

The term “a vapor stream consisting essentially of propylene oxide” asused in the context of the present invention with regard to the vaporstream resulting from (iii) relates to a vapor stream having a solventcontent of not more than 500 ppm, preferably not more than 200 ppm, morepreferably not more than 100 ppm, more preferably not more than 50 ppm,more preferably not more than 20 ppm, particularly preferably not morethan 15 ppm and very particularly preferably not more than 10 ppm, ineach case based on the total weight of the vapor stream resulting from(iii).

Compared to the processes of the prior art, an advantage of the processof the present invention is, inter alia, that it can be carried out atlower reflux ratios. According to the present invention, these refluxratios are preferably in the range of from 4 to 10. Preferred refluxratios are, among others, 4, 5, 6, 7, 8, 9 or 10.

The present invention is illustrated by the following examples.

EXAMPLES Comparative Example 1 Conventional Fractionation to Give aPropylene Oxide Fraction Having a Methanol Content of 20 ppm

Using the process disclosed in WO 00/07965, propylene oxide is preparedfrom propene by reaction with hydrogen peroxide in methanol as solventusing a titanium zeolite catalyst of the TS-1 type. Propylene oxide isseparated off by distillation from the mixture having the compositiondescribed below obtained after removal of the excess propene in such away that the resulting propylene oxide fraction has a methanolconcentration of 20 ppm.

Composition of the mixture to be separated:

Low boilers: 0.2% by weight

Propylene oxide: 9.3% by weight

Methanol: 70% by weight

Methoxypropanols: 0.5% by weight

Water: 18% by weight

High boilers: 2% by weight

The distillation is carried out at a pressure of 1.5 bar, so that thevapor can be condensed in the condenser by means of available riverwater at a temperature of about 42° C. The methanol concentration of thepropylene oxide stream is 20 ppm. The optimized separation proceeds at areflux ratio of 18 and has an energy consumption for the aboveseparation of 2 650 kWh/t (propylene oxide fraction).

Comparative Example 2 Conventional Fractionation to Give a PropyleneOxide Fraction Having a Methanol Content of 10 ppm

Using the process disclosed in WO 00/07965, propylene oxide is preparedfrom propene by reaction with hydrogen peroxide in methanol as solventusing a titanium zeolite catalyst of the TS-1 type. Propylene oxide isseparated off by distillation from the mixture having the compositiondescribed below obtained after removal of the excess propene in such away that the resulting propylene oxide fraction has a methanolconcentration of 10 ppm.

Composition of the mixture to be separated:

Low boilers: 0.2% by weight

Propylene oxide: 9.3% by weight

Methanol: 70% by weight

Methoxypropanols: 0.5% by weight

Water: 18% by weight

High boilers: 2% by weight

The distillation is carried out at a pressure of 1.5 bar, so that thevapor can be condensed in the condenser by means of available riverwater at a temperature of about 42° C. The methanol concentration of thepropylene oxide stream is 10 ppm. The optimized separation proceeds at areflux ratio of 23 and has an energy consumption for the aboveseparation of 3 280 kWh/t (propylene oxide fraction).

Example 1 Fractionation According to the Present Invention to Give aPropylene Oxide Fraction Having a Methanol Content of 20 ppm

Using the process disclosed in WO 00/07965, propylene oxide is preparedfrom propene by reaction with hydrogen peroxide in methanol as solventusing a titanium zeolite catalyst of the TS-1 type. Propylene oxide isseparated off by distillation from the mixture having the compositiondescribed below obtained after removal of the excess propene in such away that the resulting propylene oxide fraction has a methanolconcentration of 20 ppm.

Composition of the mixture to be separated:

Low boilers: 0.2% by weight

Propylene oxide: 9.3% by weight

Methanol: 70% by weight

Methoxypropanols: 0.5% by weight

Water: 18% by weight

High boilers: 2% by weight

The distillation is carried out at a pressure of 500 mbar using a vaporcompressor and using the compressed vapor for operating the naturalconvection vaporizer of the column. The vapor is compressed to apressure of about 2.8 bar by means of a turbocompressor (single-stageturbine compressor having 2×4 rotors), resulting in the compressed vaporhaving a temperature of about 68° C. The electric energy consumption ofthe compressor is about 6 MW. The vaporization temperature in theconvection vaporizer of the column is about 54° C.

The reflux ratio in this separation (residual methanol concentration of20 ppm in the propylene oxide stream) is about 8. The additional energyrequirement for the separation in addition to that introduced by thermalcoupling is about 180 kWh/t (propylene oxide fraction).

Example 2 Fractionation According to the Present Invention to Give aPropylene Oxide Fraction Having a Methanol Content of 10 ppm

Using the process disclosed in WO 00/07965, propylene oxide is preparedfrom propene by reaction with hydrogen peroxide in methanol as solventusing a titanium zeolite catalyst of the TS-1 type. Propylene oxide isseparated off by distillation from the mixture having the compositiondescribed below obtained after removal of the excess propene in such away that the resulting propylene oxide fraction has a methanolconcentration of 10 ppm.

Composition of the mixture to be separated:

Low boilers: 0.2% by weight

Propylene oxide: 9.3% by weight

Methanol: 70% by weight

Methoxypropanols: 0.5% by weight

Water: 18% by weight

High boilers: 2% by weight

The distillation is carried out at a pressure of 500 mbar using a vaporcompressor and using the compressed vapor for operating the naturalconvection vaporizer of the column. The vapor is compressed to apressure of about 2.8 bar by means of a turbocompressor (single-stageturbine compressor having 2×4 rotors), resulting in the compressed vaporhaving a temperature of about 68° C. The electric energy consumption ofthe compressor is about 8 MW. The vaporization temperature in theconvection vaporizer of the column is about 54° C.

The reflux ratio in this separation (residual methanol concentration of10 ppm in the propylene oxide stream) is about 9. The additional energyrequirement for the separation in addition to that introduced by thermalcoupling is about 200 kWh/t (propylene oxide fraction).

1-10. (canceled)
 11. A process for preparing propylene oxide, whichcomprises at least the steps (iii) and (iv) (iii) separating offpropylene oxide from a mixture (M1) comprising propylene oxide and atleast one solvent by distillation in a distillation column, giving abottom stream and a vapor stream consisting essentially of propyleneoxide; (iv) compressing the vapor stream obtained in (iii) by means ofat least one compressor to give a compressed vapor.
 12. A process asclaimed in claim 11, wherein the at least one solvent is methanol.
 13. Aprocess as claimed in claim 11, wherein the distillation column used forthe separation by distillation in (iii) is operated at a pressure in therange of from 450 to 750 mbar.
 14. A process as claimed in claim 11,wherein the compression of the vapor is carried out using aturbocompressor.
 15. A process as claimed in claim 11, wherein the vaporis compressed to a pressure in the range of from 2 to 5 bar in (iv) andthe compressed vapor has a temperature which is in a range of from 8 to20° C. above the temperature of the medium vaporizing in thedistillation column in (iii).
 16. A process as claimed in claim 11,which additionally comprises the step (v) (v) condensing the vaporobtained in (iv) and returning at least part of the heat of condensationto at least one vaporizer used in the distillation column employed in(iii).
 17. A process as claimed in claim 16, which additionallycomprises the step (vi) (vi) cooling at least part of the condensateobtained in (v) to a temperature in the range of from 10 to 30° C. in atleast one heat exchanger and returning this part of the cooledcondensate as reflux to the distillation column used in (iii).
 18. Aprocess as claimed in claim 17, wherein propene compressed in the atleast one heat exchanger used in (vi) is vaporized completely withdepressurization.
 19. A process as claimed in claim 11, wherein theenergy stored in the bottom stream obtained in (iii) is at least partlyused for heating the mixture (M1) before it is fractionally distilled in(iii).
 20. A process as claimed in claim 11, which additionallycomprises the steps (i) and (ii) (i) reacting propene with hydrogenperoxide in the presence of a titanium silicalite catalyst and methanolas solvent to give a mixture (M0) comprising propylene oxide, unreactedpropene and methanol; (ii) separating off the unreacted propene from themixture (M0) to give a mixture (M1) comprising propylene oxide andmethanol.
 21. A process for preparing propylene oxide, which comprisesat least the steps (iii) and (iv) (iii) separating off propylene oxidefrom a mixture (M1) comprising propylene oxide and at least one solventby distillation in a distillation column, giving a bottom stream and avapor stream consisting essentially of propylene oxide; (iv) compressingthe vapor stream obtained in (iii) by means of at least one compressorto give a compressed vapor, wherein the at least one solvent ismethanol.
 22. A process as claimed in claim 21, wherein the distillationcolumn used for the separation by distillation in (iii) is operated at apressure in the range of from 450 to 750 mbar.
 23. A process as claimedin claim 21, wherein the compression of the vapor is carried out using aturbocompressor.
 24. A process as claimed in claim 21, wherein the vaporis compressed to a pressure in the range of from 2 to 5 bar in (iv) andthe compressed vapor has a temperature which is in a range of from 8 to20° C. above the temperature of the medium vaporizing in thedistillation column in (iii).
 25. A process as claimed in claim 21,which additionally comprises the step (v) (v) condensing the vaporobtained in (iv) and returning at least part of the heat of condensationto at least one vaporizer used in the distillation column employed in(iii).
 26. A process as claimed in claim 25, which additionallycomprises the step (vi): (vi) cooling at least part of the condensateobtained in (v) to a temperature in the range of from 10 to 30° C. in atleast one heat exchanger and returning this part of the cooledcondensate as reflux to the distillation column used in (iii).
 27. Aprocess as claimed in claim 26, wherein propene compressed in the atleast one heat exchanger used in (vi) is vaporized completely withdepressurization.
 28. A process for preparing propylene oxide, whichcomprises at least the steps (iii) and (iv) (iii) separating offpropylene oxide from a mixture (M1) comprising propylene oxide and atleast one solvent by distillation in a distillation column, giving abottom stream and a vapor stream consisting essentially of propyleneoxide; (iv) compressing the vapor stream obtained in (iii) by means ofat least one compressor to give a compressed vapor, which additionallycomprises the steps (i) and (ii) (i) reacting propene with hydrogenperoxide in the presence of a titanium silicalite catalyst and methanolas solvent to give a mixture (M0) comprising propylene oxide, unreactedpropene and methanol; (ii) separating off the unreacted propene from themixture (M0) to give a mixture (M1) comprising propylene oxide andmethanol, and the steps (v) and (vi) (v) condensing the vapor obtainedin (iv) and returning at least part of the heat of condensation to atleast one vaporizer used in the distillation column employed in (iii),and (vi) cooling at least part of the condensate obtained in (v) to atemperature in the range of from 10 to 30° C. in at least one heatexchanger and returning this part of the cooled condensate as reflux tothe distillation column used in (iii).